If you've ever complained about having bad sex, you really have no idea. Human women may have to complain about poor stamina or incompetent technique but the female seed beetle (or bean weevil; Callosobruchus maculatus) has to contend with her partner's nightmarish penis - an organ covered in hard, sharp spikes. Just see if you can look at the picture on the right without wincing.
It's no surprise then that females sustain heavy injuries during sex. But why have male beetles evolved such hellish genitals? What benefits do they gain by physically harming their partners?
It's possible that the injuries directly benefit the males, either because they stop the females from mating again or spend more efforts in raising their fertilised eggs to avoid the strain of future liaisons.
The alternative is that the spikes could give the males an edge in "sperm competitions", where they compete with rivals not through direct combat, but through fertilising as many eggs as possible. In this theory, the spines are important for winning these competitions, and the wounds they inflict are simply a nasty side-effect.
Cosima Hotzy and Goran Arnqvist from Uppsala University think that the latter theory is right. They have found that the penile spines are vital to a male's success - those with the longest spikes fertilise the most eggs and father the most young. Size, it seems, really does matter.
The duo studied beetles taken from 13 countries across the tropics, from Brazil to the USA, and from Nigeria to Oman. The genitals of these different populations are very varied and to study them under a microscope, Hotzy and Arnqvist first had to fluff their subjects. They anaesthetised the males with carbon dioxide, and erected their penises with an "artificial inflater" - a microscopic plastic tip connected to a pump. Under a microscope, they measured the length of the longest spines and the size of the entire spine-bearing area.
They studied female beetles too, and all from a single population. Each one was allowed to mate once with a single male. A week later, Hotzy and Arnqvist dissected their sexual tracts to see how much scarring they had, using the distinctive black pigments on the scars to spot them. As you might expect, the males with the spiniest genitals inflicted the most amount of scarring on the females, in a way that was independent of the partners' overall body size.
To measures the effects of these spines on the males' reproductive success, the duo first mated virgin females with males from a single Nigerian population, who had all been sterilised by radiation. Two days later, the females were then paired with a second male, taken from one of the 13 worldwide populations. One week later, they counted all the eggs she laid. Every one of these must have been fathered by the second male, so their number represents his ability to successfully oust the sterile sperm of the first partner.
Again, the males with the longest penile spines were more likely to be the victors of the sperm competition and again, this link had nothing to do with either the male's body size or his general health. The degree of scarring in the females was also linked to the male's success at sperm competition, but to a much lesser extent and certainly not when the length of the spines was taken into account. It is the spines, rather than the damage they inflict, that is the key to the male's success.
It's not entirely clear what exactly the spines do. They could anchor the male during sex, position his penis in the right spot during sex, or scrape out the sperm of rivals from the female's genitals. Either way, they have clearly evolved to allow the male seed beetle to out-ejaculate his rivals.
This fierce competition for fertilisation rights is fairly common among insects and has led to the evolution of some truly horrific sexual strategies. The seed beetle is far from the only example of what Hotzy and Arnqvist call "sexually antagonistic co-evolution".
Some species, like the dung fly, also have armoured genitals. Others, like the familiar fly Drosophila melanogaster, have toxic semen laden with chemicals that actually slash the lifespans of females, who can literally mate their way to death. But even here, the effect of this lethal cocktail isn't the early death of the female; that's an unfortunate side-effect. Their goal is to encourage the female to lay more eggs or reject further suitors, or even kill the sperm of other males.
And possibly the most cringeworthy sexual technique of all belongs to the male bed bug, whose penis is like a hypodermic needle. He stabs it through the female's back and injects his semen straight into her abdominal cavity. It's a method that's been appropriately named "traumatic insemination".
Now, for anyone who has managed to stick this through (and believe me, I've been wincing with every keystroke), female insects have evolved their own defences. The female seed beetle, for example, has a huge amount of connective tissue lining her sexual tracts so that the damage inflicted by the males doesn't affect her too badly. She also takes a more direct approach to protecting herself - when she's had enough, she simply kicks the male until he lets go. Doing so cuts the total length of mating by a third - this is one species where the females want the males to ejaculate sooner.
Reference: Cosima Hotzy, GÃ¶ran Arnqvist (2009). Sperm Competition Favors Harmful Males in Seed Beetles Current Biology DOI: 10.1016/j.cub.2009.01.045
Image: from Nature
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That was really fascinating!
Lets see the kooks try to attribute this to intelligent design by a loving god :)
Don't female bed bugs have two vaginas - a real one and a fake, cul-de-sac sort of thing? I think this is the male's reasoning behind the traumatic insemination. Ouch.
Yeah, it's called a "spermalege", but I think it's a counter-adaptation to traumatic insemination, rather than the driver for it. See these two papers, both open-access.
Wow, Thank you Ed for toughing this one out and bringing this paper, with some of the amazing, if painful to ponder, sexual adaptations in insects in it, to us all.
Of course to be fair, there are many, many sexual adaptations in the animal kingdom, and many of those have the insect female superior, and there are some adaptations in vertebrate sex that would make a sailor scream and clutch his member in horror. (Or maybe not...)
Still that mace-for-a-penis adaptation is, ouch!
New reader here... This is very interesting and I really enjoy your style. Keep up the good work.
I've heard that Placoderms had the same thing. Still...ouch.
For more weird (and hopefully less stomach-churning) animal sex acts, check out this post from Oh For the Love of Science.
Assuming they can feel pain. Thought I don't doubt they can.
Does this explain human genital piercings?
Woohoo! Ed covered sex! I've heard that the spines have a secondary ability to scoop out the sperm of other males, is there evidence of that?
This is hard to look at. Question: how sure are the researchers that these spiny projections are, um, stiff and (presumably chitinously) hard? I see reference to "scarring", but if these structures aren't stiff and hard to begin with wouldn't these structures be relatively, um, 'soft and floppy applicators' upon, um, deployment and act to enhance fertilization without the need for increasing the risk of infection in the inseminated female? Just wondering.
SciC - I think that's the case for some insects, and it's been suggested for the seed beetle too. However, I think one of the other references I saw discounted this idea (although I can't remember why).
Astrounit - they're pretty convinced of the hardness. I think the exact word is "sclerotised" which is an arthropod-specific term that refers to a hardening of the exoskeleton. There's no flaccid stage.
Whoa. That makes it even harder to contemplate. No pun intended.
I'm so glad the election is over. I can imagine what Sarah Palin and John McCain would have to say about this research.
If these scientists are ever at a cocktail party, how do they answer the question "So, what have you been studying lately?"
"Beetle penis length and it evolutionary implications. Quite fascinating actually. The biggest problem we had was getting the male erect. First, we had to knock him out--little bastards get really upset when you start sticking a ruler in his privates. We tried that GHB stuff but come to find out, ordinary CO2 dos this trick quite nicely.
"Then we had to get him erect. We tried one of those penis vacuum pumps. But jimmny cricket, those pumps actually make his, ah, member 25% bigger. Who woulda thought? I've been using one for years and its never worked on me. But it seems the beetle penis is quite responsive. Quite interesting but clearly not worth publishing.
"Well, we finally stumbled on a design that works (from Thailand, doncha know). Then the real work began. I'd measure penis length day-in day-out for months. I started dreaming about beetle penis length. Do you remember that movie Superbad? Dude, I was that kid but it was beetle penises!"
Actually, this is more common than one might think as male cats also have spiny penises (hence all the yowling during mating season). It's theorized that the spines especially hurt/scar the female if they try to pull away from the male. So this is basically a way for the male to secure a sexual position and "spread his seed."
It's explained better here, http://en.wikipedia.org/wiki/Cat#Reproduction.
Yeah, that's quite the phallus there... All I can really say is that I'm glad I'm a human right now! :)
Terrific research and article. There are fancy condoms with ribs, bumps, and spikes that claim to enhance female pleasure. They almost look as horrible as the beetle penis--some medieval instrument of torture. Fortunately for humans, penis size does not determine reproductive success. There is no evidence that men with longer penises are more likely to fertilize a female partner. Semen is projectile ejaculated, but even if deposited inside by a short penis, the sperm swim. Because human sexual desire continues even during infertile days in the menstrual cycle, human mating is obviously intended for more than procreation. It encourages pairing. Also, female sexual pleasure receptors are fairly accessible. The clitoris is essentially external. The G-spot is only 2-3 inches up the anterior wall of the vagina. Most nerve endings are also clustered close to the entrance of the vagina. so the ability of the female to get sexual pleasure from mating encourages her acceptance of a male regardless of penis size. On the other hand, humans have evolved with a disproportionately large penis size among primates, so that would imply that natural selection among our humanoid ancestors preferred males with bigger penis size. But today, in spite of societal hype that bigger is better--in everything--surveys of female preference in penis size indicate that most women prefer an average sized penis. Larger ones may cause pain, although not the kind of damage that the beetle penis would, of course. Human sexuality is much more complex than that of insects. Attractiveness includes the preference of the male being a good provider --a good hunter, in caveman days. so women surveyed also seem reluctant to reject a short-penised partner if he was satisfactory in other ways. Presumably or hopefully he'd have sexual skills included in his attributes. smallpenisgreatlover.com is a site which addresses these issues.
This is all very...terrifying...